Method for making light emitting diode

ABSTRACT

A method for making light emitting diode is provided. The method includes following steps. A light emitting diode chip is provided, the light emitting diode includes a first semiconductor layer, an active layer and a second semiconductor layers stacked on a surface of a substrate in that order. A patterned mask layer is located on the second semiconductor layer, and the patterned mask layer includes a number of bar-shaped protruding structures aligned side by side. The second semiconductor layer is etched to form a number of three-dimensional nano-structures preform. The mask layer is removed to form a number of M-shaped three-dimensional nano-structures. The second semiconductor layer and the active layer are etched to expose a portion of the first semiconductor layer. A first electrode is electrically connected with the first semiconductor layer. A second electrode is electrically connected with the second semiconductor layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201110293093.8, filed on Oct. 7, 2011 in the China Intellectual Property Office, disclosure of which is incorporated herein by reference. This application is related to applications entitled, “METHOD FOR MAKING LIGHT EMITTING DIODE”, filed Dec. 29, 2011 Ser. No. 13/340,653; “LIGHT EMITTING DIODE”, filed Dec. 29, 2011 Ser. No. 13/340,627; “METHOD FOR MAKING LIGHT EMITTING DIODE”, filed Dec. 29, 2011 Ser. No. 13/340,658; “LIGHT EMITTING DIODE”, filed Dec. 29, 2011 Ser. No. 13/340,661; “LIGHT EMITTING DIODE”, filed Dec. 29, 2011 Ser. No. 13/340,643.

BACKGROUND

1. Technical Field

The present disclosure relates to a light emitting diode (LED) and the method for making the same.

2. Description of the Related Art

LEDs are semiconductors that convert electrical energy into light. Compared to conventional light sources, the LEDs have higher energy conversion efficiency, higher radiance (i.e., they emit a larger quantity of light per unit area), longer lifetime, higher response speed, and better reliability. At the same time, LEDs generate less heat. Therefore, LED modules are widely used as light sources in optical imaging systems, such as displays, projectors, and so on.

A conventional LED commonly includes an N-type semiconductor layer, a P-type semiconductor layer, an active layer, an N-type electrode, and a P-type electrode. The active layer is located between the N-type semiconductor layer and the P-type semiconductor layer. The P-type electrode is located on the P-type semiconductor layer. The N-type electrode is located on the N-type semiconductor layer. Typically, the P-type electrode is transparent. In operation, a positive voltage and a negative voltage are applied respectively to the P-type semiconductor layer and the N-type semiconductor layer. Thus, holes in the P-type semiconductor layer and electrons in the N-type semiconductor layer can enter the active layer and combine with each other to emit visible light.

However, extraction efficiency of LEDs is low because typical semiconductor materials have a higher refraction index than that of air. Large-angle light emitted from the active layer may be internally reflected in LEDs, so that a large portion of the light emitted from the active layer will remain in the LEDs, thereby degrading the extraction efficiency.

What is needed, therefore, is a light emitting diode and a method for making the same, which can overcome the above-described shortcomings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 shows a schematic view of one embodiment of an LED.

FIG. 2 is an isometric view of one embodiment of a three-dimensional nano-structures array of the LED of FIG. 1.

FIG. 3 shows a Scanning Electron Microscope (SEM) image of the three-dimensional nano-structures array of FIG. 2.

FIG. 4 shows a cross-sectional view along a line IV-IV of FIG. 2.

FIG. 5 is a figure that shows the luminous efficiency of one embodiment of an LED with a three-dimensional nano-structure array and the luminous efficiency of an LED without any three-dimensional nano-structure array.

FIG. 6 is a flowchart of one embodiment of a method for making an LED.

FIG. 7 is a flowchart of one embodiment of forming a three-dimensional array of FIG. 6.

FIG. 8 shows a process of one embodiment of forming a plurality of three-dimensional nano-structure preforms in the method of FIG. 7.

FIG. 9 is a SEM image of a plurality of three-dimensional nano-structure preforms of FIG. 8.

FIG. 10 shows a schematic view of one embodiment of an LED.

FIG. 11 is a flowchart of one embodiment of a method for making an LED.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one.

Referring to FIG. 1, an LED 10 includes a substrate 100, a first semiconductor layer 110, an active layer 120, a second semiconductor layer 130, a first electrode 112, a second electrode 132, and a three-dimensional nano-structure array 140. The first semiconductor layer 110, the active layer 120, and the second semiconductor layer 130 are stacked on the substrate 100 in that order, and the first semiconductor layer 110 is contacting the substrate 100. The first electrode 112 is electrically connected with the first semiconductor layer 110, and the second electrode 132 is electrically connected with the second semiconductor layer 130. The three-dimensional nano-structure array 140 is formed on a surface of the second semiconductor layer 130 away from the substrate 100.

The substrate 100 can be made of a transparent material and adapted to support the first semiconductor layer 110. A shape or a size of the substrate 100 is according to need. The substrate 100 includes an epitaxial growth surface 101 which is used to grow the first semiconductor layer 110. The epitaxial growth surface 101 is a clean and smooth surface. The substrate 100 can be a single-layer structure or a multi-layer structure. If the substrate 100 is a single-layer structure, the substrate 100 can be a single crystal structure having a crystal face used as the epitaxial growth surface 101. If the substrate 100 is a multi-layer structure, the substrate 100 should include at least one layer having the crystal face. The material of the substrate 100 can be GaAs, GaN, AlN, Si, SOL SiC, MgO, ZnO, LiGaO₂, LiAlO₂, or Al2O₃. The material of the substrate 100 can be selected according to the material of the first semiconductor layer 110. The first semiconductor layer 110 and the substrate 100 should have a small crystal lattice mismatch and a thermal expansion mismatch. The size, thickness, and shape of the substrate 100 can be selected according to need. In one embodiment, the substrate 100 is a sapphire substrate in a thickness of about 40 nm.

The first semiconductor layer 110 is located on the epitaxial growth surface 101. The first semiconductor layer 110 is an N-type semiconductor or a P-type semiconductor. The material of the N-type semiconductor can include N-type gallium nitride, N-type gallium arsenide, or N-type copper phosphate. The material of the P-type semiconductor can include P-type gallium nitride, P-type gallium arsenide, or P-type copper phosphate. The N-type semiconductor is configured to provide photons, and the P-type semiconductor is configured to provide holes. The thickness of the first semiconductor layer 110 ranges from about 1 μm to about 5 μm. In one embodiment, the first semiconductor layer 110 is an N-type gallium nitride doped with Si. The first semiconductor layer 110 includes a first surface and an opposite second surface. The first surface is contacting the substrate 100. The second surface includes a first region and a second region. The first region is used to support the active layer 120 and the second semiconductor layer 130, and the second region is used to support the first electrode 112.

The active layer 120 and the second semiconductor layer 130 are stacked on the first region in that order. The active layer 120 covers the entire surface of the first region. The active layer 120 is a photon excitation layer and can be one of a single layer quantum well film or multilayer quantum well films. The active layer 120 is made of GaInN, AlGaInN, GaSn, AlGaSn, GalnP, or GalnSn. In one embodiment, the active layer 120 has a thickness of about 0.3 μm and includes one layer of GaInN and another layer of GaN. The GaInN layer is stacked with the GaN layer.

The first electrode 112 is electrically connected with the first semiconductor layer 110. The first electrode 112 covers at least part of the surface of the second region. The first electrode 112 is spaced from the active layer 120. The first electrode 112 is a single layer structure or a multi-layer structure. The first electrode 112 can be an N-type electrode or a P-type electrode according the first semiconductor layer 110. The material of the first electrode 112 can be selected from Ti, Ag, Al, Ni, Au, or any combination of them. The material of the first electrode 112 can also be indium-tin oxide (ITO) or carbon nanotube film. In one embodiment, the first electrode 112 is a two-layer structure consisting of a Ti layer with about 15 nm in thickness and an Au layer with about 100 nm in thickness.

The second semiconductor layer 130 is located on the active layer 120. In one embodiment, the second semiconductor layer 130 covers the entire surface of the active layer 120. The thickness of the second semiconductor layer 130 ranges from about 0.1 μm to about 3 μm. The second semiconductor layer 130 can be an N-type semiconductor layer or a P-type semiconductor layer. Furthermore, the type of the second semiconductor layer 130 is different from the type of the first semiconductor layer 110. A surface of the second semiconductor layer 130 is used as a light emitting surface of the LED 10. In one embodiment the second semiconductor layer 130 is a P-type gallium nitride doped with Mg.

Further referring to FIG. 2, one embodiment of the three-dimensional nano-structure array 140 includes a plurality of three-dimensional nano-structures 142. The plurality of the three-dimensional nano-structures 142 is aligned side by side with a certain interval to form an array, concentric circles, or concentric rectangles. The distance between two adjacent three-dimensional nano-structures 142 can range from about 0 nanometers to about 1000 nanometers, such as 20 nanometers or 30 nanometers. The plurality of three-dimensional nano-structures 142 extends substantially along the same direction to form an array on the second semiconductor layer 130. The distance between the two adjacent three-dimensional nano-structures 142 is about 10 nanometers.

The three-dimensional nano-structures 142 can be a protruding structure. The protruding structure protrudes out from the surface of the second semiconductor layer 130. The material of the three-dimensional nano-structures 142 can be the same as or different from that of the second semiconductor layer 130. The three-dimensional nano-structure 142 can be attached on the surface of the second semiconductor layer 130, and the three-dimensional nano-structure 142 can also be integrated with the second semiconductor layer 130 to form an integrated structure.

The plurality of three-dimensional nano-structures 142 can be arranged side by side. Each three-dimensional nano-structure 142 can extend substantially along a straight line, a curvy line, or a polygonal line. The extending direction is substantially parallel with the surface of the second semiconductor layer 130. The two adjacent three-dimensional nano-structures are arranged with a certain interval. The distance ranges from about 0 nanometers to about 200 nanometers. The extending direction of the three-dimensional nano-structure 142 can be fixed or varied. If the extending direction of the three-dimensional nano-structure 142 is fixed, the plurality of three-dimensional nano-structures 142 extends along a straight line. Otherwise, the three-dimensional nano-structures 142 extends along a polygonal line or a curvy line. In one embodiment, each three-dimensional nano-structure 142 is a bar-shaped protruding structure extending along a straight line as shown in FIG. 3. The plurality of three-dimensional nano-structures 142 are substantially parallel with each other and extend substantially along the same direction to form an array. The plurality of three-dimensional nano-structures 142 are uniformly and equidistantly distributed on the entire surface of the second semiconductor layer 130.

Further referring to FIG. 4, the extending direction of the three-dimensional nano-structure 142 is defined as the X direction, and the Y direction is substantially perpendicular to the X direction and substantially parallel with the surface of the second semiconductor layer 130. The three-dimensional nano-structure 142 extends from one side of the second semiconductor layer 130 to the opposite side of the second semiconductor layer 130 along the X direction. The three-dimensional nano-structure 142 is a double-peak structure including two peaks. The cross-section of the double-peak structure is M-shaped. The three-dimensional nano-structure 142 having an M-shaped cross-section is referred as an M-shaped three-dimensional nano-structure 142 hereinafter. Each M-shaped three-dimensional nano-structure 142 includes a first peak 1422 and a second peak 1424. The first peak 1422 and the second peak 1424 extend side by side along the X direction. The first peak 1422 includes a first surface 1422 a and a second surface 1422 b. The first surface 1422 a and the second surface 1422 b intersect to form an intersection line and an included angle θ of the first peak 1422. The intersection line can be a straight line, a curvy line, or a polygonal line. The included angle θ is greater than 0 degree and smaller than 180 degrees. In one embodiment, the included angle θ ranges from about 30 degrees to about 90 degrees. The first surface 1422 a and the second surface 1422 b can be planar, curvy, or wrinkly. In one embodiment, the first surface 1422 a and the second surface 1422 b is planar. The first surface 1422 a intersects the surface of the second semiconductor layer 130 at an angle α. The angle α is greater than 0 degrees and less than or equal to 90 degrees. In one embodiment, the angle α is greater than 80 degrees and less than 90 degrees. The first surface 1422 a includes a side connected to the surface of the second semiconductor layer 130, and extends away from the second semiconductor layer 130 to intersect with the second surface 1422 b. The second surface 1422 b includes a side connected with the second peak 1424, and extends away from the second semiconductor layer 130 with an angle β. The angle β is greater than 0 degrees and less than 90 degrees.

The second peak 1424 includes a third surface 1424 a and a fourth surface 1424 b. The structure of the second peak 1424 is the same as that of the first peak 1422. The third surface 1424 a and the fourth surface 1424 b intersect each other to form the included angle of the second peak 1424. The third surface 1424 a includes a side intersected with the surface of the second semiconductor layer 130, and extends away from the second semiconductor layer 130 to connect with the fourth surface 1424 b. The fourth surface 1424 b includes a side intersected with the third surface 1424 a to form the included angle of the second peak 1424, and extends to intersect with the second surface 1422 b of the first peak 1422 to define a first groove 1426. A second groove 1428 is defined between two adjacent three-dimensional nano-structures 142. The second groove 1428 is defined by the third surface 1424 a of the second peak 1424 and the first surface 1422 a of the first peak 1422 of the adjacent three-dimensional nano-structure 142.

The first peak 1422 and the second peak 1424 protrude out of the semiconductor layer 130. The height of the first peak 1422 and that of the second peak 1424 is arbitrary and can be selected according to need. In one embodiment, both the height of the first peak 1422 and that of the second peak 1424 range from about 150 nanometers to about 200 nanometers respectively. The height of the first peak 1422 can be equal to that of the second peak 1424. Both the first peak 1422 and the second peak 1424 have a highest point respectively. The highest point of the first peak 1422 and the second peak 1424 is defined as the farthest point away from the surface of the semiconductor layer 130. In one three-dimensional nano-structure 142, the highest point of the first peak 1422 is spaced from that of the second peak 1424 with a certain distance ranging from about 20 nanometers to about 100 nanometers. The first peak 1422 and the second peak 1424 extend substantially along the X direction. The cross-section of the first peak 1422 and the second peak 1424 can be a trapezoid or a triangle, and the shape of the first peak 1422 and the second peak 1424 can be the same. In one embodiment, the cross-section of the first peak 1422 and the second peak 1424 is a triangle. The first peak 1422 and the second peak 1422 form the double-peak structure. In one embodiment, the first peak 1422, the second peak 1424 and the second semiconductor layer 130 form an integrated structure. Because of technology limitations, the first surface 1422 a and the second surface 1422 b cannot be absolutely planar.

In each M-shaped three-dimensional nano-structure 142, the first peak 1422 and the second peak 1424 define the first groove 1426. The extending direction of the first groove 1426 is substantially the same as the extending direction of the first peak 1422 and the second peak 1424. The cross-section of the first groove 1426 is V-shaped. The depth h₁ of the first groove 1426 in different three-dimensional nano-structures 142 is the same. The depth h₁ is defined as the distance between the highest point of the first peak 1422 and the bottom of the first groove 1426. The depth of the first groove 1426 is less than the height of the first peak 1422 and the second peak 1424.

The plurality of three-dimensional nano-structures 142 is aligned side by side and substantially parallel with each other. The second groove 1428 is defined between the two adjacent three-dimensional nano-structures 142. The second groove 1428 extends along the extending direction of the three-dimensional nano-structures 142. The cross-section of the second groove 1428 is V-shaped or an inverse trapezium. Along the extending direction, the cross-section of the second groove 1428 is the same. The depth h₂ of the second grooves 1428 between each two adjacent three-dimensional nano-structures 142 is substantially the same. The depth h₂ is defined as the distance between the highest point and the bottom of the second groove 1428. The depth h₁ of the second groove 1428 is greater than that of the first groove 1426, and the ratio between them ranges from about 1:1.2 to about 1:3 (1:1.2≦h₁:h₂≦1:3). The depth of the first groove 1426 ranges from about 30 nanometers to about 120 nanometers, and the depth of the second groove 1428 ranges from about 90 nanometers to about 200 nanometers. In one embodiment, the depth of the first groove 1426 is about 80 nanometers, and the depth of the second groove 1428 is about 180 nanometers. The depth of the first groove 1426 and the second groove 1428 can be selected according to need.

The width of the three-dimensional nano-structure 142 ranges from about 100 nanometers to about 200 nanometers. The width of the three-dimensional nano-structure 142 is defined as the maximum span of the three-dimensional nano-structure 142 extending along the Y direction. This span gradually decreases along the direction away from the substrate 100. Thus, in each three-dimensional nano-structure 142, the distance between the highest of the first peak 1422 and that of the second peak 1424 is less than the width of the three-dimensional nano-structure 142. The plurality of three-dimensional nano-structures 142 can be distributed with a certain interval, and the interval can be substantially the same. The interval forms the second groove 1428. The distance λ₀ between the two adjacent three-dimensional nano-structures 142 ranges from about 0 nanometers to about 200 nanometers. The distance between each two adjacent three-dimensional nano-structures 142 can be substantially the same. The distance λ₀ can increase or decrease with the increase or decrease of the height of the first peak 1422 and the second peak 1424. In the Y direction, the distance λ₀ can be increased, decreased, or periodical varied. If the distance λ₀=0, the cross-section of the second groove 1428 is V-shaped. If the distance λ₀>0, the cross-section of the second groove 1428 is an inverse trapezium.

Along the Y direction, the plurality of three-dimensional nano-structures 142 is distributed in a certain period P. One period P is defined as the width λ of the three-dimensional nano-structures 142 added with the distance λ₀. The period P of the plurality of three-dimensional nano-structures 142 can range from about 100 nanometers to about 500 nanometers. The period P, the width λ, and the distance λ₀ satisfy the following formula: P=λ+λ₀. The period P, the width λ, and the distance λ₀ is measured in nanometers. The period P can be a constant, and λ₀ or λ can be a dependent variable. Furthermore, one part of the three-dimensional nano-structures 142 can be aligned in a first period, and another part of the three-dimensional nano-structures 142 can be aligned in a second period. In one embodiment, the period P is about 200 nanometers, the width λ is about 190 nanometers, and the distance λ₀ is about 10 nanometers.

The second electrode 132 can be an N-type electrode or a P-type electrode. The type of the second electrode 132 is the same as the second semiconductor layer 130. The shape of the second electrode 132 is arbitrary and can be selected according to need. The second electrode 132 covers at least part of the surface of the second semiconductor layer 130. The second electrode 130 covers at least part of the three-dimensional nano-structures 142. The second electrode 132 is located on the light emitting surface of the LED 10, and the shape and the location of the second electrode 132 cannot affect the light extraction efficiency of the LED 10. If the second electrode 132 is transparent, the second electrode 132 can cover the entire light emitting surface, thus the current diffusion speed will be improved and the heat produced in the LED will be decreased. The second electrode 132 is a single layer structure or a multi-layer structure. The material of the second electrode 132 can be selected from Ti, Ag, Al, Ni, Au or any combination of them. The material of the second electrode 132 can also be ITO. In one embodiment, the second electrode 132 is transparent to reduce the reflectivity and the absorption, thus improving the light extraction efficiency.

Furthermore, a reflector (not shown) can be located on the surface of the substrate 100. The material of the reflector can be selected from titanium (Ti), silver (Ag), aluminum (Al), nickel (Ni), gold (Au) or any combination thereof. The reflector includes a smooth surface having a high reflectivity. The photons reach the reflector and will be reflected by the reflector, thus these photons can be extracted out of the LED 10, and the light extraction efficiency of the LED 10 can be improved.

The plurality of three-dimensional nano-structures 142 change the motion direction of the photons reaching the light emitting surface with a large incident angle, so that these photons can be extracted from the light emitting surface. Furthermore, because the three-dimensional nano-structure 142 is M-shaped, the M-shaped three-dimensional nano-structures 142 can function as two layers of the three-dimensional nano-structure assembled together. The light extraction efficiency of the LED 10 will be improved. Referring to FIG. 5, the light extraction efficiency of the LED 10 is 5 times higher than that of an LED without any three-dimensional nano-structures.

Referring to FIG. 6, a method for making the LED 10 includes following steps:

(S11), providing a substrate 100 having an epitaxial growth surface 101;

(S12), forming an LED chip by growing a first semiconductor layer 110, an active layer 120 and a second semiconductor layer 130 on the epitaxial growth surface 101 in that order;

(S13), forming a three-dimensional nano-structure array 140 on the surface of the second semiconductor layer 130;

(S14), applying a first electrode 112 electrically connected to the first semiconductor layer 110;

(S15), applying a second electrode 132 electrically connected to the second semiconductor layer 130.

In step (S11), the epitaxial growth surface 101 is a clean and smooth surface. The material of the substrate 100 can be selected according to the material of the first semiconductor layer 110. The first semiconductor layer 110 and the substrate 100 should have a small crystal lattice mismatch and a thermal expansion mismatch. The size, thickness, and shape of the substrate 100 can be selected according to need. In one embodiment, the substrate 100 is a sapphire substrate.

In step (S12), the first semiconductor layer 110, the active layer 120 and the second semiconductor layer 130 can be grown respectively via a process of molecular beam epitaxy (MBE), chemical beam epitaxy (CBE), vacuum epitaxy, low temperature epitaxy, choose epitaxy, liquid phase deposition epitaxy (LPE), metal organic vapor phase epitaxy (MOVPE), ultra-high vacuum chemical vapor deposition (UHVCVD), hydride vapor phase epitaxy (HVPE), and metal organic chemical vapor deposition (MOCVD).

In one embodiment, the first semiconductor layer 110 is Si-doped N-type GaN. The first semiconductor layer 110 is made by a MOCVD method, and the growth of the first semiconductor layer 110 is a heteroepitaxial growth. In the MOCVD method, the nitrogen source gas is high-purity ammonia (NH₃), the carrier gas is hydrogen (H₂), the Ga source gas is trimethyl gallium (TMGa) or triethyl gallium (TEGa), and the Si source gas is silane (SiH₄). The growth of the first semiconductor layer 110 includes the following steps:

(S121) placing the substrate 100 with the first carbon nanotube layer 110 thereon into a reaction chamber and heating the substrate 100 to about 1100° C. to about 1200° C., introducing the carrier gas, and baking the substrate 100 for about 200 seconds to about 1000 seconds;

(S122) growing the low-temperature GaN layer by reducing the temperature to a range from about 500° C. to 650° C. in the carrier gas atmosphere, and introducing the Ga source gas and the nitrogen source gas at the same time;

(S123) stopping the flow of the Ga source gas in the carrier gas and nitrogen source gas atmosphere, increasing the temperature to a range from about 1100° C. to about 1200° C., and maintaining the temperature for about 30 seconds to about 300 seconds;

(S124) growing the high quality first semiconductor layer 110 by maintaining the temperature of the substrate 100 in a range from about 1000° C. to about 1100° C., and reintroducing the Ga source gas again and the Si source gas.

In step (S122), the low-temperature GaN is used as a buffer layer (not shown) to grow the first semiconductor layer 110. The thickness of the buffer layer is less than the thickness of the first carbon nanotube layer 110. Because the first semiconductor layer 110 and the substrate 100 have different lattice constants, the buffer layer is used to reduce the lattice mismatch during the growth process, thus the dislocation density of the first semiconductor layer 110 will decrease.

The growth method of the active layer 120 is similar to the first semiconductor layer 110. In one embodiment, the indium source gas is trimethyl indium. The growth of the active layer 120 after the growth of the first semiconductor layer 110 includes the following steps:

(a1) stopping the flow of the Si source gas and maintaining the temperature of the reaction chamber to a range from about 700° C. to about 900° C., the pressure of the reaction chamber ranging from about 50 torrs to about 500 torrs; and

(a2) introducing the indium source gas and growing an InGaN/GaN multilayer quantum well film to form the active layer 120.

The second semiconductor layer 130 is grown after the growth of the active layer 120. In one embodiment, the Mg source gas is ferrocene magnesium (Cp₂Mg), and the method includes the following steps:

(b1) stopping the flow of the indium source gas, maintaining the temperature of the reaction chamber in a range from about 1000° C. to about 1100° C., and maintaining the pressure of the reaction chamber in a range from about 76 torrs to about 200 torrs; and

(b2) introducing the Mg source gas and growing P-type gallium nitride doped with Mg to form the second semiconductor layer 130.

Also referring to FIG. 7, in step (S13), the three-dimensional nano-structure array 140 is formed on the surface of the second conductor layer 130 by the following steps:

(S131), locating a mask layer 103 on the surface of the second semiconductor layer 130;

(S132), patterning the mask layer 103 by a nanoimpinting and etching method;

(S133), forming a plurality of three-dimensional nano-structure preforms 1421 on the surface of the second semiconductor layer 130; and

(S134), removing the mask layer 103.

In step (S131), the mask layer 103 can be a single layered structure or a multi-layered structure. The thickness of the mask layer 103 can be selected according to the etching depth or the etching atmosphere. Hereafter the pattern formed in the mask layer 103 have high precision. If the mask layer 103 is a single layered structure, the material of the mask layer 103 can be ZEP520A which is developed by Zeon Corp of Japan, HSQ (hydrogen silsesquioxane), PMMA (Polymethylmethacrylate), PS (Polystyrene), SOG (silicon on glass) and other silitriangle oligomers. The mask layer 101 is used to protect a portion of the second semiconductor layer 130. In one embodiment, the mask layer 103 is a multi-layered structure. The mask layer 103 includes a first mask layer 1032 and a second mask layer 1034 stacked on the second semiconductor layer 130 in that order, with the second mask layer 1034 covering the first mask layer 1032. The first mask layer 1032 and the second mask layer 1034 can be selected according to need. The material of the first mask layer 1032 can be ZEP520A, PMMA, PS, SAL601 and ARZ720. The material of the second mask layer 1034 can be HSQ, SOG and other silitriangle oligomers. The second mask layer 1034 can be easily printed by a mechanical method to ensure the precision of the mask layer 103. In one embodiment, the material of the first mask layer 1032 is ZEP520A, and that of the second mask layer 1034 is HSQ. The first mask layer 1032 and the second mask layer 1034 can be formed by a screen printing method or a deposition method.

The step (S131) includes sub-steps of:

(S131 a), forming the first mask layer 1032; and

(S131 b), forming the second mask layer 1034.

In the step (S131 a), the first mask layer 1032 is formed by the following steps. First, the second semiconductor layer 130 is cleaned in a clean room. Second, a layer of positive electron-beam resist can be spin-coated on the second semiconductor layer 130 at a speed of about 500 rounds per minute to about 6000 rounds per minute, for about 0.5 minutes to about 1.5 minutes. The positive electron-beam resist can be ZEP520A resist. Third, the second semiconductor layer 130 with the positive electron-beam resist can be dried at a temperature of about 140 degrees centigrade to 180 degrees centigrade, for about 3 minutes to about 5 minutes, thereby forming the first mask layer 1032 on the second semiconductor layer 130. The thickness of the first mask layer 1032 can be in a range of about 100 nanometers to about 500 nanometers.

In the step (S131 b), the mask layer 1034 can be a layer of HSQ resist. The HSQ resist is spin-coated on the first mask layer 1032 under high pressure at a speed of about 2500 rounds per minute to about 7000 rounds per minute, for about 0.5 minutes to about 2 minutes. The thickness of the second mask layer 1032 can range from about 100 nanometers to about 300 nanometers. The HSQ can be pressed to be deformed at room temperature. Moreover, the HSQ has good structural stability, and provides a high resolution often better than 10 nm.

Furthermore, a transition layer (not shown) can be deposited on the first mask layer 1032 before the step of forming the second mask layer 1034. In one embodiment, the transition layer can be a glassy silicon dioxide film with a thickness of about 10 nanometers to about 100 nanometers. The transition layer is used to protect the first mask layer 1032 while nanoimprinting the second mask layer 1034.

In step (S132), the mask layer 103 is patterned by the following method:

(a), providing a patterned template 200;

(b), attaching the template 200 on the second mask layer 1034, and imprinting the template 200 to form a plurality of slots on the second mask layer 1032;

(c), removing the residual second mask layer 1034 in the bottom of the slot to expose the first mask layer 1032; and

(d), patterning the mask layer 103 by removing one part of the first mask layer 1032 corresponding with the slots.

In step (a), the template 200 can be made of rigid materials, such as nickel, silicon, and carbon dioxide. The template 200 can also be made of flexible materials, such as PET, PMMA, polystyrene (PS), and polydimethylsiloxane (PDMS). The template 200 can be fabricated through an electron beam lithography method with the nano-pattern formed therein. The template 200 includes a plurality of protruding structures. The protruding structures are substantially parallel with each other and spaced from each other to form an array, concentric circles, or concentric rectangles. A slot is defined between the two adjacent protruding structures. The protruding structures form the nano-pattern of the template 200. The nano-pattern can be designed according to the actual application. In one embodiment, the protruding structures are bar-shaped extending substantially along the same direction. The width of the protruding structure and that of the slot can be the same. In one embodiment, both the width of the protruding structure and that of the slot range are from about 50 nanometers to about 200 nanometers.

In step (b), the template 200 is then pressed towards the second semiconductor layer 130 at room temperature. During this process, the protruding structures are pressed into the second mask layer 1034 to form a plurality of slots in the second mask layer 1034, and some materials of the second mask layer 1034 remain at the bottom of the slot. Finally, the template 200 is removed with the nano-pattern remaining in the second mask layer 1034. The nano-pattern of the second mask layer 1034 includes a plurality of second protruding structures and a plurality of slots. The protruding structures in the second mask layer 1034 correspond to the slots in the template 200. The slots in the second mask layer 1034 correspond to the protruding structures in the template 200.

In one embodiment, the template 200 is pressed towards the second semiconductor layer 130 at room temperature in a vacuum environment of about 1×10⁻¹ millibars to about 1×10⁻⁵ millibars. The pressure applied on the template 200 is about 2 pounds per square foot to about 100 pounds per square foot. The pressure is applied on the template 200 for about 2 minutes to about 30 minutes. There may be material of the second mask layer 1034 remaining at the bottom of the slots.

In step (c), the residual material of the second mask layer 1034 at the bottom of the slots can be removed by plasma etching. In one embodiment, a CF₄ reactive plasma etching method can be used to remove the material of the second mask layer 1034 remaining at the bottom of the slots. For example, the second semiconductor layer 130 with the protruding structures and the slots formed in the second mask layer 1034 can be placed in a CF₄ reactive plasma etching system. The CF₄ reactive plasma etching system generates CF₄ plasma, and the CF₄ plasma moves towards the second mask layer 1034. The material of the second mask layer 1034 remaining at the bottom of the slots will be etched away, so that the first mask layer 1032 corresponding to the slots will be exposed. At the same time, the width of the top of the protruding structures in the second mask layer 1034 is decreased during the etching process, but the nano-pattern in the second mask layer 1034 will be maintained.

In step (d), the first mask layer 1032 exposed from the slots can be removed by oxygen plasma etching. For example, the second semiconductor layer 130 after step (d) can be placed in an oxygen plasma etching system. The power of the oxygen plasma etching system can be in a range of about 10 watts to about 150 watts. The speed of the oxygen plasma can range from about 2 sccm to about 100 sccm. The partial pressure of the oxygen plasma can range from about 0.5 Pa to about 15 Pa. The etching time can range from about 5 seconds to about 1 minute. During the process of etching the first mask layer 1032, the first mask layer 1032 exposed by the slots will be removed, and the second semiconductor layer 130 corresponding to the slots will be exposed. The protruding structures in the second mask layer 1034 function as a mask to the oxygen plasma to ensure the resolution of the first mask layer 1032.

During the etching process, the pattern in the second mask layer 1034 will be copied into the first mask layer 1032 to form a patterned mask layer 103. The patterned mask layer 103 includes a plurality of protruding structures 1031 on the surface of the second semiconductor layer 130. Each protruding structure 1031 includes the first mask layer 1032 and the second mask layer 1034 stacked together. A slot 1033 is defined between each two adjacent protruding structures 1031, and the surface of the second semiconductor layer 130 which correspond to the slot 1033 is exposed. During the process of etching the first mask layer 1032, the top of the protruding structures of the second mask layer 1034 will also be partly etched. The etching speed to the second mask layer 1034 is much less than that of the first mask layer 1032, and the nano-pattern in the second mask layer 1034 can still be maintained. Thus the resolution of the mask layer 103 can be improved.

In step (S13), the second semiconductor layer 130 after step (S12) can be placed in an inductively coupled plasma device to etch the second semiconductor layer 130 exposed by the mask layer 103. The etching gas can be selected according to the material of the second semiconductor layer 130 and the mask layer 103. During the etching process, the surface of the second semiconductor layer 130 exposed by the slots 1033 of the mask layer 103 will be etched, thereby forming a plurality of grooves in the second semiconductor layer 130.

Referring to FIG. 8 and FIG. 9, the etching process of the second semiconductor layer 130 includes the following stages:

first stage, forming a plurality of grooves with the same depth by etching the surface of the second semiconductor layer 130;

second stage, continuing the etching process so that every two adjacent protruding structures 1031 begin to slant face to face to form a protruding pair; and

third stage, continuing the etching process so that the two adjacent protruding structures 1031 gradually slant until the tops of the two adjacent protruding structures 1031 are closed.

In the first stage, the etching gas etches the exposed surface of the second semiconductor layer 130 to form a plurality of grooves. The grooves have substantially the same depth because of the same etching speed.

In the second stage, during the etching process, the etching gas will react with the second semiconductor layer 130 to form a protective layer. The protective layer will reduce the etching speed to the second semiconductor layer 130, and the width of the grooves will slowly decrease from the outer surface of the second semiconductor layer 130 to the bottom of the grooves. Thus, the inner wall of the grooves will not be absolutely perpendicular to the surface of the second semiconductor layer 130, but form an angle. Furthermore, the etching does not only etch the second semiconductor layer 130, but also etch the top of the protruding structures 1031. The width of the top of the protruding structures 1031 will decrease. The resolution of the mask layer 103 will not be affected because the etching speed of the top of the protruding structures 1031 is far less than that of the second semiconductor layer 130. Furthermore, each two adjacent protruding structures 1031 will slant face to face.

In the third stage, the tops of the two adjacent protruding structures 1031 will gradually approach each other. The speed of etching the base 101 corresponding to these closed two adjacent protruding structures 1031 will decrease, and the width of the grooves will gradually decrease from the outer surface of the base 101 to the bottom of the grooves. Because the two adjacent protruding structures 1031 slant face to face to form protruding pair, the speed of etching the base 101 corresponding to the protruding pair will further decrease. Eventually, the tops of the two adjacent protruding structures 103 close, and the etching gas can no longer etch the base 101 corresponding to the two adjacent protruding structures 103, thus the first groove 1426 is formed on the surface of the base 101. But between every two adjacent protruding pairs, the etching speed will change less than that of the slant two adjacent protruding structures 1031. Thus the second grooves 1428 is formed, and the depth of the second grooves 1428 will be greater than that of the first grooves 1426, thereby obtaining the plurality of three-dimensional nano-structure preforms 1021 on the substrate 100.

In one embodiment, the etching gas includes Cl₂, BCl₃, O₂, and Ar. The power of the inductively coupled plasma device ranges from about 10 watts to about 100 watts, the flow speed of the etching gas ranges from about 8 sccm to about 150 sccm, the pressure of the etching gas can range from about 0.5 Pa to about 15 Pa, and the etching time can range from about 5 seconds to about 5 minutes. The flow speed of the Cl₂ ranges about 2 sccm to about 60 sccm, the flow speed of the BCl₃ ranges from about 2 sccm to about 30 sccm, the flow speed of the O₂ ranges from about 3 sccm to about 40 sccm, and the flow speed of the Ar ranges from about 1 sccm to about 20 sccm. In one embodiment, the flow speed of the etching gas ranges from about 40 sccm to about 100 sccm to improve the resolution and the etching speed. In another embodiment, the power of the inductively coupled plasma device is about 70 watts, the flow speed of the etching gas is about 40 sccm, the pressure of the etching gas is about 2 Pa, and the etching time is about 2 minutes. In the etching gas, the flow speed of the Cl₂ is about 26 sccm, the flow speed of the BCl₃ is about 16 sccm, the flow speed of the O₂ is about 20 sccm, and the flow speed of the Ar is about 10 sccm.

The mask layer 103 and the etching gas is not limited. The etching gas includes one gas or a mixture mixed of different gases, so long as the top of the two adjacent protruding structures 1031 in the mask layer 103 can close. The flow speed of the etching gas, the pressure, the etching time, and the ratio between the different gases can be selected according to the three-dimensional nano-structures 142.

In step (S134), the mask layer 103 can be removed by dissolving in a stripping agent such as tetrahydrofuran (THF), acetone, butanone, cyclohexane, hexane, methanol, or ethanol. In one embodiment, the stripping agent is butanone, and the mask layer 103 is dissolved in butanone and separated from the second semiconductor layer 130. The mask layer 103 is removed to form the plurality of three-dimensional nano-structures 142 located on the second semiconductor layer 130. The plurality of three-dimensional nano-structures 142 and the second semiconductor layer 130 are integrated to an integrated structure.

The three-dimensional nano-structure array 140 can also be formed by locating another dielectric layer (not shown) on the second semiconductor layer 130 and etching the dielectric layer. The three-dimensional nano-structure array 140 is located on the surface of the second semiconductor layer 130. The dielectric layer can be transparent to increase the light extraction efficiency of LED 10.

In step (S14), the first electrode 112 can be formed by the following steps:

S141, exposing a portion of the surface of the first semiconductor layer 110 by etching the second semiconductor layer 130 and the active layer 120; and

S142, locating the first electrode 112 on the exposed portion of the first semiconductor layer 110.

In step (S141), the second semiconductor layer 110 and the active layer 120 can be etched via light etching, electronic etching, plasma etching, or a chemical corrosion method.

In step (S142), the first electrode 112 can be formed via a process of physical vapor deposition, such as electron beam evaporation, vacuum evaporation, ion sputtering, or any physical deposition. Furthermore, the first electrode 112 can also be formed by directly attaching a conductive sheet on the exposed portion of the first semiconductor layer 110. The first electrode 112 is located on the second region and spaced from the active layer 120 and the second semiconductor layer 130.

In step (S15), the method for making the second electrode 132 is the same as that of the first electrode 112. The second electrode 132 is located on a portion of the surface of the second semiconductor layer 130. The location of the second electrode 132 cannot affect the light extraction efficiency of the LED 10. The second electrode 132 is a continuous layered-structure. Because the three-dimensional nano-structure array 140 is located on the surface of the second semiconductor layer 130, a portion of the second electrode 132 is deposited into the grooves in the three-dimensional nano-structure 142.

The method for making the LED 10 has following the advantages. First, the nano-imprinting method can be carried out at room temperature, and the template does not need pre-treatment, thus the method is simple and low cost. Second, the plurality of M-shaped three-dimensional structures can be easily formed on the light emitting surface of LED 10, and the productivity of the patterned substrate can be improved.

Referring to FIG. 10, an LED 20 includes a first semiconductor layer 110, an active layer 120, a second semiconductor layer 130, a first electrode 112, a second electrode 132, and a three-dimensional nano-structure array 140. The active layer 120 is sandwiched between the first semiconductor layer 110 and the second semiconductor layer 130. The first electrode 112 is electrically connected with the first semiconductor layer 110, and the second electrode 132 is electrically connected with the second semiconductor layer 130. The three-dimensional nano-structure array 140 is formed on a surface of the semiconductor layer 120, away from the active layer 130. The structure of LED 20 is similar to that of the LED 10, except that the substrate 100 is omitted and the first electrode 112 is located on the surface the first semiconductor layer 110 away from the active layer 120.

The first electrode 112 can cover the entire surface of the first semiconductor layer 110 away from the active layer 120. Thus the LED 20 forms a vertical structure, and the current diffusion speed will be improved and the heat produced in the LED 20 will be decreased.

Furthermore, a reflector (not shown) can be sandwiched between the first electrode 112 and the first semiconductor layer 110. The reflector is conductive and electrically connected with the first electrode 112 and the first semiconductor layer 110. The reflector includes a smooth surface having a high reflectivity. The photons reaching the reflector will be reflected by the reflector, thus these photons can be extracted out of the LED 20 from the light emitting surface, and the light extraction efficiency of the LED 20 can be improved.

Referring to FIG. 11, one embodiment of a method for making LED 20 includes the following steps:

S21, providing a substrate 100 having an epitaxial growth surface 101;

S22, forming an LED chip by growing a first semiconductor layer 110, an active layer 120, and a second semiconductor layer 130 on the epitaxial growth surface 101 in that order;

S23, forming a three-dimensional nano-structure array 140 on the surface of the second semiconductor layer 130;

S24, removing the substrate 100 to expose a surface of the first semiconductor layer 110;

S25, applying a first electrode 112 on the surface of the first semiconductor layer 110; and

S26, applying a second electrode 132 electrically connected to the second semiconductor layer 130.

The method of making LED 20 is similar to that of the LED 10, except that a step of removing the substrate 100 in the method of making LED 20 is performed. The substrate 100 can be removed by laser irradiation, etching, or thermal expansion and contraction. The removal method can be selected according to the material of the substrate 100 and the first semiconductor layer 110. In one embodiment, the substrate 100 is removed by laser irradiation. The substrate 100 can be removed from the first semiconductor layer 110 by the following steps:

(S241) polishing and cleaning the surface of the substrate 100 away from the first semiconductor layer 110;

(S242) placing the substrate 100 on a platform (not shown) and irradiating the substrate 100 and the first semiconductor layer 110 with a laser; and

(S243) immersing the substrate 100 into a solvent and removing the substrate 100.

In step (S241), the substrate 100 can be polished by a mechanical polishing method or a chemical polishing method to obtain a smooth surface. Thus the scatting of the laser will be decreased. The substrate 100 can be cleaned with hydrochloric acid or sulfuric acid to remove the metallic impurities and oil.

In step (S242), the substrate 100 is irradiated by the laser from the polished surface, and the incidence angle of the laser is substantially perpendicular to the surface of the substrate 100. The wavelength of the laser is selected according to the material of the first semiconductor layer 110 and the substrate 100. The energy of the laser is smaller than the bandgap energy of the substrate 100 and larger than the bandgap energy of the first semiconductor layer 110. Thus the laser can pass through the substrate 100 and reach the interface between the substrate 100 and the first semiconductor layer 110. The buffer layer at the interface has a strong absorption of the laser, and the temperature of the buffer layer will be raised rapidly. Thus the buffer layer will be decomposed. In one embodiment, the bandgap energy of the first semiconductor layer 110 is about 3.3 ev, and the bandgap energy of the substrate 100 is about 9.9 ev. The laser is a KrF laser, the wavelength of the laser is about 248 nm, the energy is about 5 ev, the pulse width range is about 20 nanoseconds to about 40 nanoseconds, the energy density ranges from about 400 mJ/cm² to about 600 mJ/cm², and the shape of the laser pattern is square with a size of 0.5 mm×0.5 mm. The laser moves from one edge of the substrate 100 with a speed of 0.5 mm/s. During the irradiating process, the GaN is decomposed to Ga and N₂. It is understood that the parameter of the laser can be adjusted according to need. The wavelength of the laser can be selected according to the absorption of the buffer layer.

Because the buffer layer has a strong absorption of the laser, the buffer layer can decompose rapidly. However, the first semiconductor layer 110 has a weak absorption, so it does not decompose quickly. The irradiating process can be performed in a vacuum or a protective gas environment. The protective gas can be nitrogen, helium, argon, or other inert gas.

In step (S243), the substrate 100 can be immersed into an acidic solution to remove the Ga decomposed from GaN so that the substrate 100 can be peeled off from the first semiconductor layer 110. The acidic solution can be hydrochloric acid, sulfuric acid, or nitric acid which can dissolve the Ga.

Depending on the embodiment, certain of the steps of methods described may be removed, others may be added, and the sequence of steps may be altered. It is also to be understood that the description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps.

It is to be understood that the above-described embodiments are intended to illustrate rather than limit the disclosure. Variations may be made to the embodiments without departing from the spirit of the disclosure as claimed. It is understood that any element of any one embodiment is considered to be disclosed to be incorporated with any other embodiment. The above-described embodiments illustrate the scope of the disclosure but do not restrict the scope of the disclosure. 

What is claimed is:
 1. A method for making a light emitting diode, comprising: providing a light emitting diode chip, wherein the light emitting diode chip comprises a first semiconductor layer, an active layer, and a second semiconductor layer stacked on a surface of a substrate in that order; locating a patterned mask layer on a surface of the second semiconductor layer, wherein the patterned mask layer comprises a plurality of bar-shaped protruding structures aligned side by side, and a slot is defined between each two adjacent protruding structures of the plurality of protruding structures to expose a portion of the second semiconductor layer; etching the exposed portion of the second semiconductor layer such that the each two adjacent protruding structures begin to slant face to face until they are contacting each other to form a protruding pair; forming a plurality of M-shaped three-dimensional nano-structures on the surface of the second semiconductor layer by removing the mask layer; exposing a portion of the first semiconductor layer by removing a part of the second semiconductor layer and the active layer; applying a first electrode to electrically connect with the first semiconductor layer on the exposed first semiconductor layer; and applying a second electrode to electrically connect with the second semiconductor layer.
 2. The method of claim 1, wherein the each two adjacent protruding structures gradually slant and tops of the each two adjacent protruding structures gradually approach each other.
 3. The method of claim 2, wherein a first part of the second semiconductor layer corresponding to each of the protruding pairs is etched at a first speed, a second part of the semiconductor layer exposed from the each of two adjacent protruding pairs is etched at a second speed, and the first speed is less than the second speed.
 4. The method of claim 3, wherein a first groove is defined in the first part of the second semiconductor layer, and a second groove is defined in the second part of the second semiconductor layer.
 5. The method of claim 4, wherein a depth of the first groove is less than a depth of the second groove.
 6. The method of claim 5, wherein the depth of the first groove ranges from about 30 nanometers to about 120 nanometers, and the depth of the second groove ranges from about 100 nanometers to about 200 nanometers.
 7. The method of claim 1, wherein the second semiconductor layer is etched in an inductively coupled plasma device, and a power of the inductively coupled plasma device ranges from about 20 watts to about 100 watts.
 8. The method of the claim 7, wherein the second semiconductor layer is etched by the following stages: etching the surface of the second semiconductor layer to form a plurality of grooves with substantially the same depth; continuing the etching process so that the each two adjacent protruding structures begin to slant face to face; and continuing the etching process so that the each two adjacent protruding structures gradually slant until the tops of the each two adjacent protruding structures are contacting each other.
 9. The method of claim 7, wherein an etching gas used in the etching process comprises Cl₂, BCl₃, O₂, and Ar.
 10. The method of claim 9, wherein a flow speed of the etching gas ranges from about 8 sccm to about 150 sccm, a pressure of the etching gas ranges from about 0.5 Pa to about 15 Pa, and an etching time ranges from about 5 seconds to about 5 minutes.
 11. The method of claim 10, wherein the flow speed of the Cl2 ranges about 2 sccm to about 60 sccm, the flow speed of the BCl3 ranges from about 2 sccm to about 30 sccm, the flow speed of the O2 ranges from about 3 sccm to about 40 sccm, and the flow speed of the Ar ranges from about 1 sccm to about 20 sccm.
 12. The method of claim 1, wherein the patterned mask layer is formed by the following steps: placing a mask layer on the surface of the second semiconductor layer; forming a plurality of bar-shaped protruding structures aligned side by side by nanoimprinting the mask layer, wherein a slot is defined between each of two adjacent protruding structures; and exposing the second semiconductor layer by etching the mask layer through the slot.
 13. The method of claim 1, wherein the patterned mask layer comprises a first mask layer and a second mask layer stacked on the second semiconductor layer in that order, and a material of the first mask layer is different from that of the second mask layer.
 14. The method of claim 13, wherein the patterned mask layer is patterned by the following steps: providing a template with a plurality of protruding structures aligned side by side and extending substantially along the same direction; attaching the template on the second mask layer; pressing the template toward the second mask layer and removing the template to form a plurality slots in the second mask layer; exposing the first mask layer by etching the second mask layer remaining on the bottom of the slots through the slots; and exposing the second semiconductor layer by etching the exposed first mask layer.
 15. The method of claim 1, wherein the plurality of three-dimensional nano-structures is periodically aligned.
 16. The method of claim 15, wherein a period of the plurality of the three-dimensional nano-structures ranges from about 100 nanometers to about 500 nanometers.
 17. A method for making a light emitting diode, comprising: providing a substrate having an epitaxial growth surface; growing a first semiconductor layer, an active layer and a second semiconductor layer on the epitaxial growth surface in that order; forming a plurality of three-dimensional nano-structures on a surface of the second semiconductor layer, wherein each of the plurality of three-dimensional nano-structures has a first peak and a second peak aligned side by side, a first groove is defined between the first peak and the second peak, a second groove is defined between each two adjacent three-dimensional nano-structures of the plurality of three-dimensional nano-structures, and a depth of the first groove is less than a depth of the second groove; applying a first electrode to electrically connect with the first semiconductor layer; and applying a second electrode to electrically connect with the second semiconductor layer. 